1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and more particularly to a method for defining structures using a combination of spacers and optical lithography.
2. Description of Related Art
In the field of the fabrication of semiconductor structures, for example during the production of integrated circuits based on semiconductor wafers, a large number of the devices on the wafers are fabricated using lithographic techniques. The lithographic techniques often employ masks or mask structures for the patterning and production of the semiconductor structures in subsequent process steps. The mask or mask structures are generally produced so that the patterns are true to size, in correct positions and orientation, and without defects.
Generally, an image of the pattern is projected onto a photosensitive material coating on a wafer. The image pattern may be either a positive or negative mask image that is projected onto the coated wafer using an optical lithography system. The optical lithography system emits radiation at a wavelength X which chemically changes the exposed areas of the coating, usually by polymerizing the coating exposed to the radiation. Depending on the solvent used, the unpolymerized areas are removed, and the desired pattern image remains.
However, as technology advances and device sizes become smaller, the need to resolve smaller image features becomes more difficult, especially since the diffraction limits of visible light wavelengths have been reached. In order to continue printing these features with high resolution and contrast, shorter wavelength radiation is needed. Typical optical lithography systems, such as steppers, may use radiation at wavelengths such as 365 nm, 248 nm, 193 nm, 157 nm, and 126 nm. However, only 193 nm steppers are commercially available for volume manufacturing, while steppers using 157 nm and 126 nm wavelengths are still being developed. Advanced non-optical lithography systems with shorter wavelengths such as extreme ultraviolet or soft x-rays are now being actively researched for printing complex patterns in submicron ranges. However, the problem of diffraction limited optics remains, and the drive to using shorter wavelengths provides only limited results.
In addition to shorter wavelength radiation, there are several techniques available for high resolution and contrast optical lithography. One technique developed uses phase-shifting masks to increase the resolution and contrast of optical lithography. Light rays transmitted through adjacent apertures of the mask follow different phases. However, phase-shifting masks are costly and difficult to manufacture because the phase structure must be closely related to specific geometries of the mask pattern. Moreover, as microcircuit pitches shrink in size, mask making techniques do not necessarily keep pace.
Another technique used is referred to as engineered illumination to help print smaller and smaller features of semiconductor microcircuits. This technique relies upon the use of various patterns of illumination including annular and quadrapole illumination and off-axis illumination. However, these require that the illuminator be extensively modified. Additionally, these methods and assist features are time consuming, expensive, and less efficient.
Other techniques involve advanced non-optical lithography systems such as extreme ultraviolet (EUV) lithography and e-beam (SCALPEL) lithography. However, these systems are currently being developed and are also cost-prohibitive.
Any shortcoming mentioned above is not intended to be exhaustive, but rather is among many that tends to impair the effectiveness of previously known techniques for fabricating semiconductor structures; however, shortcomings mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.